Why the thumbs down?
Extrasolar planets
Main article: Extrasolar planet
Of the 243 extrasolar planets discovered by June 2007, most have masses which are comparable to or larger than Jupiter's.[29] Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12,[30] the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized,[31] and a planet orbiting Gliese 876 that is estimated to be about 6 to 8 times as massive as the Earth and is probably rocky in composition.
It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets.
Several projects have been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. The NASA Terrestrial Planet Finder was one such program, but (as of 2006-02-06) this program has been put on indefinite hold. The ESA is considering a comparable mission called Darwin. The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.
Interstellar "planets"
Main article: Interstellar planetary mass object
Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space. Some scientists have argued that such objects found roaming in deep space should be classed as "planets". However, many others argue that only planemos that directly orbit stars should qualify as planets, preferring to use the terms "planetary body", "planetary mass object" or "planemo" for similar free-floating objects (as well as planetary-sized moons). The IAU's working definition on extrasolar planets takes no position on the issue. The discoverers of the bodies mentioned above decided to avoid the debate over what constitutes a planet by referring to the objects as planemos. However, the original IAU proposal for the 2006 definition of planet favoured the star-orbiting criterion, although the final draft avoided the issue.
For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos". However, recent analysis[32] of the objects has determined that their masses are each greater than 13 Jupiter-masses, making the pair brown dwarfs.[33]